1. Field of the Invention
The subject durable catalytic burner system is generally directed to a system for combustively oxidizing an inflowing mixture of fuel and air (referred to herein simply as a fuel stream) to generate heat. More specifically, the subject durable catalytic burner system incorporates a catalytic bed assembly which, at its steady state, operates in a flameless catalytic mode to effect heat-releasing oxidation reactions of the fuel stream. The subject durable catalytic burner system incorporates a combination of mechanical features which collectively yield thermodynamic efficiencies that afford the catalytic bed assembly a longer operational life.
There is a need in numerous applications for burner systems that are operable for extended periods of continuous use. In electric power generator systems located at remote, unmanned stations, for example, burner systems are employed which reactively consume an inflowing stream of fuel to generate heat that, then, is thermoelectrically converted to electric power. Typically, the burner system is mounted directly to a thermoelectric conversion unit for this purpose. The heat generated by the burner system""s operation is transferred through its heat exchanger portion to the thermoelectric conversion unit for appropriate transduction.
Catalytic burner systems are often employed in these and other such applications for the thermodynamic advantages inherent to their steady state operation. A conventional catalytic burner system 1 known and typically used in the prior art is shown in FIGS. 5 and 6. System 1 includes a housing 10 in which a burner chamber 12 is formed. In the walls surrounding this chamber 12 are an upstream opening 14 and a downstream opening 16. A fuel supply stream is introduced to and exhausted from chamber 12 through these openings 14, 16 as indicated by the directional arrows 15. Housing 10 is formed of a metallic or other suitable material such that the walls and floor defining burner chamber 12 serve collectively as a heat exchanger that effectively transfers the heat generated within chamber 12 to a thermoelectric conversion or other such unit mounted therebeneath.
Disposed within chamber 12 is a catalytic bed assembly 20 that, upon sufficient initial heating, catalyzes oxidation of the fuel/fair mixture constituting the introduced stream of fuel to sustain a level of generated heat. The assemblyxe2x80x94which is supported in part by a plurality of heat conductive posts 18 projecting upward from the floor of burner chamber 12xe2x80x94includes upstream and downstream mesh retaining members 22, 24. Mesh retaining members 22, 24 serve as fuel-pervious retaining wall structures between which a bed of catalytic bead members 26 are retained.
While not shown, a cover is typically installed directly over chamber 12. Such cover is coupled to housing 10, so as to fit tightly against catalytic bed assembly 20 and thereby prevent the incoming fuel stream from bypassing that catalytic bed assembly 20.
Briefly, operation of system 1 occurs as follows. As the fuel stream traverses catalytic bed assembly 20, it is initially ignited within that chamber 12, downstream of catalytic bed assembly 20. As the resulting flame burns within chamber 12, the individual catalytic bead members 26 are gradually heated until enough of them attain a sufficient temperature to catalyze a flameless oxidation reaction o f at least a portion of the fuel stream. Enough of the fuel in the stream is eventually consumed in this manner that an insufficient concentration of fuel remains to sustain the flame combustion. The initially ignited flame thus extinguishes, and flameless catalytic combustion prevails, whereby the catalytic bead members 26 are maintained in their sufficiently heated state by heat released from the ongoing catalyzed oxidation reactions. The region of most intense oxidationxe2x80x94thus, of most intense heat productionxe2x80x94then propagates upstream through catalytic bed assembly 20 until the upstream-most layer of catalytic bead members 26 come to oxidize much of the fuel in the passing fuel stream.
While adequate for basic operation, such prior art catalytic burner systems are encumbered by a number of shortcomings. First, its mechanical features permit the premature degradation of catalytic bead members 26, permitting in turn the premature degradation of the burner system""s thermodynamic efficiency. Each type of catalyst composition (typically, coated onto the surface of a ceramic or other suitable substrate to form catalytic bead members 26) that may be employed for catalytic bed 20 is characterized by a range of temperatures at which it serves its catalyzing function in stable manner. At temperatures above this range, a given catalyst composition becomes unstable and sustains a measurable damage if maintained at the excessive temperatures. Even within its range of temperatures, a catalyst composition""s ultimate durability is closely correlated with the temperatures at which it is maintained during burner operation. Generally, the lower the temperature at which a catalyst composition is maintained during operation, the longer its useful life. Conversely, the higher the temperature at which a catalyst composition is maintained during operation, the quicker it degrades. Particularly in applications requiring extended periods of burner operation, therefore, it becomes important to minimize the catalyst composition""s operating temperature within the permissible range. Adequate measures to so minimize the catalyst composition""s temperature are not provided in catalytic burner systems heretofore known utilized as sources of heat.
The sectional or transverse area of the catalyst bed""s upstream side, or xe2x80x98facexe2x80x99 is found to be an important factor in this context. An increase in the transverse area yields a corresponding increase in the spatial distribution of the total heat produced by the catalyzed reaction. Increasing the transverse area consequently affords a lower operating temperature for each individual catalyst bead member within a catalytic bed. Moreover, as it is the upstream-most transverse layer of catalytic bead members 26 that first reacts with the stream of fuel impinging thereon, the transverse area at the upstream face of the catalytic bed proves to be of particular importance.
When subjected to substantial periods of normal use, many of the beads 26 forming the bed""s upstream-most portions in the prior art burner system 1 are visibly degraded, having lost a substantial proportion of their catalytic capacity. A disproportionately greater degradation is typically revealed in catalytic bead members 26 at the upstream-most regions of the catalytic bed than at the downstream-most regions. In long term operation, the catalytic bed""s upstream-most layer degrades in catalytic performance until it becomes inactive, causing the next layer of bead members 26 to become the most active. This continues, in turn, for successive layers of bead members 26, such that the catalytic bed is progressively destroyed from its upstream-most to its downstream-most portions, until its catalytic performance is diminished beyond acceptable levels.
The problem is aggravated where a concentration of flow occurs at the catalytic bed""s upstream-most portions. Variations in flow resistance in the bed cause the flow to be more concentrated along certain stream paths through the catalytic bed. Directional arrows 17 and 19 illustrate examples of stream paths potentially of differing flow concentration.
Another shortcoming found in the prior art catalytic burner system 1 is that of inefficient catalytic bed heating during the initial phases of burner operation. The initially ignited flame bears against the downstream face of the catalytic bed to provide the required bed heating. Without measures to intensify the heat of the flame, it is not uncommon in many applications for insufficient heating of the catalytic bed (to enable self-sustaining catalytic mode operation) to occur before the flame is squelched due to diminishing fuel concentration. This disrupts the transition between the flame and catalytic modes of operation, ultimately causing system failure.
It is found using platinum coated alumina beads as the catalyst composition, for example, that maintaining a flame stable enough to ensure proper transition to the catalytic combustion mode of operation necessitates catalyst temperatures in at least a portion of the bed to reach excessive levels. Platinum coated alumina beads are rendered sufficiently active to serve their catalyzing function at approximately 900xc2x0 F., and are capable of withstanding a maximum temperature of 1200xc2x0 F. over extended periods of time. Such beads exhibit instability at temperatures exceeding that maximum; yet, the conditions required to maintain a stable flame for the catalytic bed heating are found to produce at certain points in the bed catalyst temperatures reaching approximately 1500xc2x0 F.
There exists a need, therefore, for a catalytic burner system that is thermodynamically efficient in operation. There also exists a need for such a catalytic burner system wherein premature degradation of the catalytic bed material is avoided, and wherein sufficient transition between flameless and catalytic combustion modes of operation reliably occurs.
2. Prior Art
Burner Systems which employ catalyst members for catalyzing flameless combustion are known in the art. The best prior art known to applicant includes U.S. Pat. Nos.: 5,993,192; 5,968,456; 5,921,769; 5,917,144; 5,842,851; 5,753,383; 5,571,484; 5,251,609; 5,161,964; 5,009,592; 4,911,143; 4,767,467; 4,726,767; 4,692,306; 4,294,225; 4,292,274; 4,235,588; 4,189,294; 4,047,876; 3,881,962; and 3,627,588. Systems disclosed in such prior art, however, fail to disclose the combination of features uniquely incorporated in the subject durable catalytic burner system. There is no catalytic burner system heretofore known which preserves thermodynamic efficiencies in the manner disclosed herein, nor is there any catalytic burner system heretofore known which prevents premature degradation of the catalyst material and reliably effects the transition between flameless and catalytic combustion modes of operation in the manner disclosed herein.
It is a primary object of the present invention to provide a catalytic burner system which is thermodynamically efficient in operation.
It is another object of the present invention to provide a catalytic burner system wherein premature degradation of the catalytic bed is avoided.
It is yet another object of the present invention to provide a catalytic burner system wherein the transition between flame and catalytic combustion modes of operation occurs reliably.
These and other objects are attained in the subject durable catalytic burner system. The subject durable catalytic burner system incorporates a catalytic bed assembly for catalyzing the oxidation reaction of an introduced stream of fuel. The catalytic bed assembly generally comprises first and second retaining members which are pervious to the fuel stream, and which define a compartment therebetween. The catalytic bed assembly also comprises a plurality of catalytic members disposed within the compartment for catalyzing flameless combustion of the fuel stream to generate heat. The first retaining member includes an upstream face portion transversely extended relative to the fuel stream to describe a convex peripheral contour about at least a portion of the compartment. The second retaining member includes a downstream face portion transversely extending relative to the fuel stream to describe a concave peripheral contour about at least a portion of the compartment. The upstream face portion is greater in surface area than the downstream face portion.
The reduction in cross-sectional area from the upstream-most to the downstream-most portions of the catalytic bed""s retaining members serves to concentrate the combustion process during the initial stages of system operation while the fuel concentration remaining in the fuel stream concurrently is decreasing. During steady state operation, the flow concentration mitigates the reduction in catalytic oxidation from lower fuel concentration, decreasing the temperature gradient from the upstream-most to the downstream-most portions of the catalytic bed. The resulting system yields enhanced combustion in the downstream portions of the catalytic bed, and thereby increases thermodynamic efficiency.
In a preferred embodiment, at least a portion of each upstream and downstream face portion describes along at least one planar dimension a cylindrically arced contour. The first and second retaining members in that embodiment are spaced at their cylindrically arced portions by a substantially uniform radial spacing, whereby a catalytic bed having a substantially uniform thickness is formed thereat. The uniformity in bed thickness provides uniformity in resistance to the fuel stream""s flow therethrough, which in turn provides uniformity in temperature within a layer of catalytic bead members transverse to the flow direction.